In recent years, battery cells based on aluminium-ion chemistry (AIB) have demonstrated their potential as a performance-dominant technology in various studies at laboratory cell size, e.g. in PAT systems. While the reported performance seems impressive, the transferability and thus comparability with other technologies is difficult.
With the first successful small format single layer pouch cell prototypes by Fraunhofer IISB (see P1-090) with approx. 20 mAh, an extensive characterisation campaign was carried out to describe the advantages and disadvantages of this new technology relative to established cell types. The cells were characterised at SOC levels of 90, 70, 50 and 30% SOC at three temperatures of 10°C, 20°C and 30°C.
Investigation of the electrochemical activity of the cells by means of cyclovoltammograms (Fig. 1) shows a very low charging capacity in the lower SOC range. In addition, the upper voltage limits are known to be unstable, with decomposition reactions of the cell components. The operating voltage range has therefore been reduced to 1.2-2.35V without significant loss of capacity, but with simplified application due to the range already known from other cell chemistries such as LTO.
The self-discharge of the manufactured cells was measured using the voltage hold method for 24 hours per voltage step. The results (Fig. 2) show self-discharge rates in the order of C/200 at higher voltages and significantly decreasing rates at lower voltages. Elevated temperatures reinforce the observed behaviour.
The measurement of the P-OCV characteristic (Fig. 3) was carried out at a relatively high current of 0.5C due to the high self-discharge rate. During the repetition of several cycles (N=4) no deviation of the voltage characteristic was observed. The voltage plateau towards the end of the charge could cause difficulties for correct balancing algorithms in later applications. Here the AIB is similar to the LFP OCV characteristic.
Investigation of the impedance by EIS (Fig. 4) shows a low correlation with SOC but a high dependence on temperature. However, the magnitude of the cell is quite small in relation to the small capacity, highlighting the power capability of the AIB.
This performance capability was analysed by evaluating the discharge behaviour at different C-rates [0.5-10]C (Fig. 5). The results indicate a rather small decrease in available capacity with increasing current rates. As shown by the error bars, the variance over 10 cycles was negligible, demonstrating the high quality of the cells manufactured. The behaviour between cells is also comparable.
Overall, the results show that, despite the upscaling of the capacity, the characteristics of the AIB known from laboratory cells have been maintained. While the relatively high self-discharge rate needs to be addressed in future work to ensure efficient use as an energy storage technology, the performance achieved indicates good suitability for high power applications such as grid integration for instantaneous back-up.